Preservation of biological samples is an important task in the workflow of both bench research and clinical applications. Typically, the study design and analysis of ex vivo tissue samples determines the method by which the specimen is processed and preserved. Tissues used for morphological or immunohistochemical analyses are frequently fixed with a common chemical fixative, such as formalin, and are embedded in paraffin. While formalin-fixed paraffin-embedded specimens are well preserved and conveniently stored at ambient room temperatures, the cross-linking and sulfide bond formation caused by the fixative make them less compatible with current molecular techniques (1-3). Hence, epitope retrieval methodologies are used to recover the antigens; but recovery in some instances can be less than optimal (1, 4, 5).
An alternative to the use of fixatives is the century-old method of freezing the tissue at sub-zero temperatures. Freezing the tissue provides a snapshot of the cells as they would appear at the time the sample was removed from the organism, while avoiding degradation of intracellular molecules via autolysis or similar mechanism (6-8). The vox populi among pathologists and histologists is to store snap-frozen tissue (aka fresh/frozen) samples, which have been sectioned and desiccated, at −80° C. Typically, sectioned tissues are placed onto microscope slides (subbed or with electrostatic charge), dehydrated to remove moisture, and immediately stored at cryogenic temperatures.
The widely accepted belief that snap-freezing any tissue adequately preserves protein integrity at cellular and sub-cellular levels has served as the gold standard for molecular analysis (1-3, 9). Ever since Altmann (7) described cellular degradation in the form of proteolysis, the conventional method of cryogenic storage of frozen tissues has been used, and few deviations from this practice have been reported. However, cryogenic storage of tissue samples is both cumbersome and costly. For example, a recent pilot study conducted at an American university demonstrated that the cost of cryogenic storage can easily run into the range of tens of millions of dollars in operating cost per year, not to mention the large carbon footprint imposed on the environment. In light of the current trend of large scale bio-banking in both clinical and research settings, this cost is likely to grow exponentially.
Therefore, there still exists a need for better methods and devices that will provide cost effective and environmentally friendly ways to store biological samples.